well aluminum oxide is white... if you use straight water, or not enough coolant, the aluminum will oxidize and leave crud like that. sometimes its caused by electrolysis, i cant remember exactly, but it happens cuz the water gets ionized and is in contact w/ dissimilar metals... just like when you have ferrous bolts going into a non-ferrous metal (steel head bolts in aluminum block) its the whole reason that Anti-seize compound was made.